Add error message for non C interoperable structs

Author: certik

src/lpython/semantics/python_ast_to_asr.cpp

@@ -2734,6 +2734,21 @@ class CommonVisitor : public AST::BaseVisitor<Struct> {
             );
             throw SemanticAbort();
         }
+        if (ASR::is_a<ASR::Struct_t>(*asr_alloc_type)) {
+            ASR::StructType_t *st = ASR::down_cast<ASR::StructType_t>(ASR::down_cast<ASR::Struct_t>(asr_alloc_type)->m_derived_type);
+            if (st->m_abi != ASR::abiType::BindC) {
+                diag.add(diag::Diagnostic(
+                    "The struct in c_p_pointer must be C interoperable",
+                    diag::Level::Error, diag::Stage::Semantic, {
+                        diag::Label("not C interoperable",
+                                {asr_alloc_type->base.loc}),
+                        diag::Label("help: add the @ccallable decorator to this struct to make it C interoperable",
+                                {st->base.base.loc}, false)
+                    })
+                );
+                throw SemanticAbort();
+            }
+        }
         fill_shape_and_lower_bound_for_CPtrToPointer();
         return ASR::make_CPtrToPointer_t(al, loc, cptr, pptr, target_shape, lower_bounds);
     }